IAS: Nappes

Nappes are the result of complex folding mechanism caused by intense Horizontal movement and resultant compressive force.

Both the limbs of a recumbent fold are parallel and horizontal.

Due to further increase in the continued compressive force, one limb of the recumbent fold slides forward and overrides the other fold. This process is called ‘thrust’ and the plane along which one part of the fold is thrust is called ‘thrust plane’

The upthrust part of the fold is called overthrust fold. Due to continuous compressive and horizontal movement, the broken limb of the fold is thrown several kilometers away from the original place. Such broken limb of the fold is called ‘nappe’

Crustal Fracture

Crustal fracture refers to displacement of rocks along a plane due to tensional and compressional forces acting either horizontally or vertically or sometimes even in both ways. Crustal fracture depends on the strength of rocks and intensity of tensional forces. Generally, fractures are divided into:

joints

faults A joint is defined as a fracture in the crustal rocks wherein no appreciable movement of rock takes place, whereas a fracture becomes fault when there is appreciable displacement of the rocks on both sides of a fracture and parallel to fault-a fault is a fracture in the crustal rocks due to tensional movement caused by the endogenetic forces. Different components of a fault:

Fault Dip: It is the angle between the fault plane and horizontal plane.

Up thrown side: It represents the uppermost block of a fault.

Downthrown side: It represents the lowermost block of a fault.

Hanging wall: It is the upper wall of a fault.

Foot wall: It represents the lower wall of a fault. Fault scarp: It is the steep wall-like slope caused by faulting of the crustal rocks.

Normal faults: These are formed due to the displacement of both the walls in opposite directions due to frac-ture. Consequently, there is great stress. The fault plane is usually between 45o and the vertical.

Reverse faults: These are formed due to the movement of both the fractured rock blocks towards each other. The fault plane in a reverse fault is usually inclined at an angle between 40° and the horizontal. Step faults: When a series of faults occur in any area in such a way that the slopes of all the fault planes of all the faults are in reverse direction, the resultant faults are called as step faults.

Rift Valley

Rift valley is a major relief feature resulting from faulting activities. Rift valley represents a trough, depression or basis between two crustal parts. Rift valleys are actually formed due to displacement of crustal parts and subsidence of middle portion between two normal faults. Rift valleys are generally also called as ‘graben’ which is a German word which means a trough-like depression. A rift valley may be formed in two ways, viz:

When the middle portion of the crust between two normal faults is dropped downward while the two blocks on the either side of the down dropped block remain stable.

When the middle portion between two normal faults remains, stable and the two side blocks on either side of the middle portion are raised upward.

Rhine rift valley is the best example of a well-defined rift valley. Death valley in the southern California (USA) The floor of the Jordan rift valley and Death sea. The Narmada valley, the Damodar valley and some stretches of the Son valley, the Tapti valley. The central plain of Scotland, Spencer Bay of Australia, etc. Are examples of rift valleys.

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